| The internal combustion engine is one of the power sources of the vehicles. It converts the thermal energy released from the combustion of fuels into the mechanical energy which drives the vehicles. The safe running of vehicles greatly depends on the operational reliability of internal combustion engine. As an important friction pair, the thermal load and lubrication friction status of piston set-cylinder liner directly influence on the reliability and durability of internal combustion engine. It plays a decisive role in the power performance, economical efficiency and safe operation of internal combustion engine. The operating conditions of piston set-cylinder liner are always severe such as high temperature, high pressure and high impact load. The circulating and transient high-temperature high-pressure gas exposes the piston set-cylinder liner to much heat load. If the temperature of heated parts in internal combustion engine is too high, the hardness and strength of material will be decreased sharply, thus possibly resulting in ablation and deformation of heated parts. Meanwhile, the lubricating oil film may be destroyed or even coked, causing serious abrasion and failure. Therefore, how to solve the problems of heat transferand lubrication friction of piston set-cylinder liner is one of hot issues which have received much attention.Many experiments have demonstrated that the addition of nanoparticles in traditional heat transfer medium can effectively strengthen the heat transfer effect. Moreover, the addition of nanoparticles in traditional lubricants can significantly improve the lubrication and friction properties. In this paper, the nano-diamond lubricants were applied to the internal combustion engine for the enhancement of heat transfer and improvement of lubrication friction. The practical application values were studied by experiments and numerical simulation. Moreover, the coupling interaction mechanism of nano-diamond lubricating oil to enhance heat transfer and improve lubrication friction was discussed. The main research contents of this paper were described as follows:(1) The fundamental experiment researches on the application of nano-diamond lubricants in internal combustion engine. Firstly, the nano-diamond lubricants were prepared by two-step method. Secondly, the transportation parameters (viscosity, thermal conductivity and specific heat capacity) of nano-diamond lubricants were studied by experiments, and then compared to the prediction model. Thirdly, the conventional friction experiments of nano-diamond lubricants were conducted to investigate the load-carrying capacity as well as the properties of anti-wear and friction-reduction. Then, the sliding friction of piston set-cylinder liner under continuously changing working conditions (varying the temperature, speed and load) was simulated to conduct the simulation studies on the sliding friction of nano-diamond lubricants. Finally, the surface morphology of friction pair was observed by scanning electron microscope. Through combining with the molecular dynamics simulation results, the physical mechanism of nano-diamond lubricating oil in the improvement of lubrication friction was analyzed and discussed.(2) The actual experiment researches on nano-diamond lubricants in internal combustion engine. The prepared nano-diamond lubricants were applied to the real internal combustion engine. The reverse dragging process tests of engine, speed characteristic test of engine, load characteristic test of engine, mapping characteristics test of engine and the temperature field test of piston set-cylinder liner were performed by the AVL bench experimental system. The using effect of real machine by nano-diamond lubricants was investigated, including the improvement of lubrication friction, the enhancement of heat transfer and the increase of fuel economy. It provided the relevant experiment basis for the numerical simulation. Meanwhile, the practical use value of nano-diamond lubricants was verified.(3) The introduction of nano-diamond lubricants to establish the coupling mathematical physical model for the heat transfer and lubrication friction of piston set-lubricating oil film-cylinder liner. Firstly, considering the impact of nano-diamond lubricating oil on heat transfer, the coupling heat transfer mathematical physical model of piston set-lubricating oil film-cylinder liner was created, mainly including the heat transfer model of sliding-contact components (piston set-cylinder liner), the physical parameter prediction model of nanofluids, the convection heat transfer model for gas-liquid phases in cooling gallery and the friction heat distribution model. Secondly, considering the iact of nano-diamond lubricating oil on lubrication friction, the mathematical physical model for unstable-thermal mixing-lubrication friction was established based on the traditional hydrodynamic lubrication theory, mainly including the equilibrium equation of piston ring, the average Reynolds equation, surface roughness, oil film thickness equation, oil film energy equation, viscosity temperature equation, asperity contact model and gas pressure model.(4) The coupling numerical calculation on the heat transfer and lubrication friction of piston set-lubricating oil film-cylinder liner in the use of nano-diamond lubricants. Firstly, the verification of coupling system in stable temperature field was conducted. The rationality and creditability of coupling mathematical physical model for heat transfer and lubrication friction were proved. Secondly, this coupling model was used for circulating and transient simulation. The coupling effect of nano-diamond lubricating oil on the heat transfer and lubrication friction (temperature field, minimum oil film thickness, friction force, friction heat, oil film temperature, oil film viscosity, etc.) of coupling system was analyzed. Thirdly, the effect of lubricating oil friction heat on the heat transfer of piston set-cylinder liner was analyzed using the simulation results. Then, the effect of application of nano-diamond lubricating oil in cooling gallery of piston on the heat transfer and lubrication friction of piston set-cylinder liner was investigated, which mainly investigated the effects of application of nano-diamond lubricating oil in cooling gallery on the heat transfer of piston set-cylinder liner and the effects of heat transfer enhancement on lubrication friction with nano-diamond lubricating oil in cooling gallery. Finally, based on the analysis results above, the coupling interaction mechanism of nano-diamond lubricating oil to enhance heat transfer and improve lubrication friction was analyzed and proposed. |
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